You are going to need a form for casting a new strut. Casting forms are typically made from
wood.
Assuming that you want to make a flanged strut, here is a simple way to make your form. Slip the old strut onto the shaft and support the shaft so that it is located correctly and runs true. Mark the strut where it intersects the hull and saw it off there, so that the top of the sawn off strut is flush with the hull bottom. Saw a couple pieces of
plywood for the flanges. Round the leading and outside edges and taper the aft edge for good
water flow. Clamp or hold the flanges against the bottom of the hull, on each side of the strut, and bond them to the strut with quick setting
epoxy. Slide it off your shaft. You will now have a form which fits your
boat bottom and locates your shaft correctly.
I would recommend that you have the casting X-rayed. I have had struts cast which had porosity (internal) at the top of the strut where it intersects the flanges. An X-ray will detect this.